On Monday, the SFO said: "Now that the investigation into the issue of regulatory misbehaviour has concluded, the SFO are considering whether it is both appropriate and possible to bring criminal prosecutions ... we hope to come to a conclusion within a month."

The investigation could be undermined by David Cameron’s proposed inquiry into the banking scandal.

Mr Cameron has said he wanted a joint committee of MPs and peers to investigate the scandal so that they can make recommendations for reform by January, rejecting Labour’s calls for a full, independent, judge-led public inquiry.

However, the Attorney General, Dominic Grieve, has warned that a speedy investigation would potentially threaten to prejudice a criminal trial.

“If there are criminal investigations or enquiries then it will be difficult,” Mr Grieve told MPs. “Any inquiry by this House will have to be managed in light of that process because it mustn’t interfere with it.”

It was reported last night that the Government is ready to boost the budget of the Serious Fraud Office so it can pursue a full inquiry into Libor.